It is well known in the prior art to provide syringes with movable stop members forming a part of the plunger rod, for the purpose of permitting the setting of the desired dosage to be dispensed. These syringes are widely used in dispensing medicaments, particularly in the veterinary field.
One common prior art design for adjusting the dosage to be dispensed employs a threaded nut cooperating with threads on the exterior surface of the plunger rod to permit the nut to be adjusted along the length of the rod to a desired, spaced position from an outer marginal surface of the syringe barrel. Upon depression of the plunger rod, for the purpose of dispensing a medicament from the syringe barrel, the nut will engage the outer margin of the barrel to control the quantity of medicament dispensed from the syringe. Typical syringes employing a rotary nut are disclosed in U.S. Pat. Nos. 475,909, issued to Wilcox; 3,563,240, issued to Silver; 4,246,898, issued to Travalent et al.; 4,153,056, issued to Silver et al.; and British patent No. 1,212,823, issued to Silver.
A problem encountered in the prior art systems employing rotating nuts, other than the system described in the Silver U.S. Pat. No. 4,513,056, is that the plunger rod of the syringe must be made in two sections, in order to permit the nut to be inserted over the rod, as it cannot fit over the plunger seal at the end which is necessarily wider than the rod. This increases the manufacturing, material and assembly costs, as compared to a system in which the plunger rod and plunger seal are made as a one-piece unit. The device disclosed in the aforementioned Silver '056 patent does not suffer from this deficiency because the ring structure is of a split construction, and is designed to be inserted over the one piece plunger rod in a direction laterally of the longitudinal axis of said rod.
In the construction disclosed in the Silver '056 patent the adjustable nut can be a one-piece, hinged element having two adjacent open end portions that can be secured to each other to form a complete annular nut. Alternatively, the adjustable member can be made of two separate pieces which can be placed around the stem and fastened together. A split ring construction has proven to be not well suited for use in dispensing highly viscous, pasty substances, wherein high pressures need to be imparted to the plunger rod to dispense the substance. Under such high pressure operation the split ring pops off the plunger rod or has actually fractured when pressed against the syringe barrel.
There are several ways disclosed for adjusting the dosage to be dispensed in the dispenser described in the Silver '056 patent. One way is to rotate the nut axially along the plunger rod, through cooperating threads on the nut and the rod. In an alternative embodiment the plunger rod can have a smooth, unthreaded surface, and the nut can be engaged with the plunger rod solely by friction, in which case the nut can be slide (i.e., without rotation) along the rod to a desired location. In this latter embodiment there is no positive means for retaining the nut in its set position to prevent undesired, inadvertent displacement of the nut along the rod. An additional mode of adjusting the nut on the rod, and one which is suggested when the position of the nut on the rod is a substantial distance from its desired position, is to physically remove the split or hinged nut from the rod and replace it adjacent its desired location. This latter method of adjusting the dosage requires the disassembly and reassembly of the nut with the rod; a procedure which is somewhat complex and undesirable.
Although the use of a solid threaded nut for setting the dosage eliminates the problems encountered with the slit ring arrangement, a solid nut presents other deficiencies which increase the cost of manufacture, assembly and use of the syringe. First, if a person wants to dispense a large percentage (e.g., 50%, or even 100%) of the material in the barrel, it is necessary to rotate the nut a substantial distance alone the piston rod, from its initial position adjacent the end of the syringe barrel, where it is required to be located initially, to lock the plunger rod so that it will not inadvertently be moved to dispense the material in the barrel. Second, when the syringe barrel is prefilled in automated equipment it is necessary to rotate the nut to the upstream end of the plunger rod, adjacent the thumb pad, so that the nut will not interfere with the filling and venting of the syringe barrel. After the syringe barrel has been filled the continuous nut must be rotated in the opposite, or downstream, direction along the plunger rod, to the marginal end of the syringe body, to thereby lock the plunger rod against inadvertent movement into the syringe barrel, which, if permitted to occur, would result in the inadvertent dispensing of the material from the barrel. The need to manipulate or adjust the nut along the plunger rod, as described above adds undesired cost to the manufacturing and assembly operations.
It also has been suggested in the prior art to employ a stop member which is adapted to be slid along the length of the rod of a syringe into a desired notch in the rod, for the purpose of setting the desired dosage to be dispensed. Representative devices of this type are disclosed in U.S. Pat. Nos. 4,642,102, issued to Ohmori, and 1,852,658, issued to Kile. These devices do not permit infinite adjustment of the dosage to be dispensed, since they require the stop member to be positively located in a preformed notch.
In summary, the prior art syringes for dispensing measured quantities of a medicament generally have been considered to be unreliable in setting and maintaining the desired dosage to be dispensed, to lack the desired flexibility in adjusting the dosage within very narrow limits, to be too cumbersome to handle in assembly before and during filling of the syringe body and to be more costly to manufacture.